1. Field of the Invention
The present invention relates to a process for producing a vinyl chloride-based polymer.
2. Description of the Prior Art
Vinyl chloride-based polymers are resins having excellent physical properties, and are used extensively as hard and soft materials for producing shaped products. As a method for processing vinyl chloride-based polymers into shaped products, there have been widely used such methods as calendering, extrusion, injection molding and the like. In connection with extrusion of hard polymers, in particular, development of a vinyl chloride-based polymer having high bulk specific gravity is desired, in order to increase the amount of resin extruded from an extruder.
For production of vinyl chloride polymers, suspension polymerization processes have been widely employed in which vinyl chloride monomer is suspended and dispersed in an aqueous medium and the resulting disperse suspension is subjected to suspension polymerization. It is known that the bulk specific gravity of a polymer obtained by the suspension polymerization is influenced by the kind of dispersing agent, stirring conditions, water-to-monomer amount ratio, and so forth. In order to produce vinyl chloride polymers with high bulk specific gravity by suspension polymerization, there have been proposed, for example, a method in which vinyl chloride monomer is additionally charged during polymerization (Refer to Japanese Pre-examination Patent Publication (KOKAI) No. 59-168008 (1984)), and a method wherein a highly saponified polyvinyl alcohol is used as a dispersing stabilizer for suspending and dispersing the vinyl chloride monomer in an aqueous medium (Refer to Japanese Pre-examination Patent Publication (KOKAI) No. 57-76008 (1982)).
The above-mentioned methods, however, have the drawback that the resulting polymers have poor porosity, leading to increased fish eyes in shaped products obtained therefrom, although the methods can produce vinyl chloride-based polymers with high bulk specific gravity. Besides, the polymers obtained by the methods show lowered rates of gelation, which causes difficulties in processing the polymers into shaped products. Further, there has been the problem of thermal stability being lowered depending on the dispersing stabilizer used.